Conventionally chromium has been plated from aqueous chromic acid baths prepared from chromic oxide (CrO.sub.3) and sulphuric acid. Such baths, in which the chromium is in hexavalent form, are characterized by low current efficiency. The chromic acid fumes emitted as a result of hydrogen evolution also present a health hazard. In order to overcome the undesirable aspects of hexavalent chromium plating, it has been proposed to plate chromium in trivalent form. One such process for plating chromium or a chromium alloy from an aqueous solution of a chromium (III)- thiocyanate complex is described in UK Pat. No. 1,431,639 and U.S. Pat. No. 4,062,737.
U.S. Pat. No. 4,161,432 describes a chromium or chromium alloy plating solution and process in which an aqueous solution of a chromium (III) thiocyanate complex is again employed but in which a buffer material supplies one of the ligands to the chromium complex. The buffer material is selected from amino acids (eg glycine), peptides, formates, acetates and hypophosphites.
Our UK Patent Application No. 13458/78 discloses a technique for avoiding deleterious anode reactions in electroplating baths by providing a perfluorinated cation exchange membrane to separate an anolyte from a catholyte. The effect on the required plating voltage of the electrical resistance of the membrane may be reduced by addition of a depolarising species to the anolyte. In one example a catholyte containing an aqueous solution of a chromium (III)-thiocyanate complex including glycine is separated by a membrane from an anolyte comprising an Agar gel saturated with a solution of 2 M potassium iodide in 0.1 M sulphuric acid. The potassium iodide is a depolarising agent and provides iodide ions (I.sup.-) which are oxidized to iodine at the anode.